The bleaching of pulps is well known in the art and many different processes are commercially utilized. Generally, the processes utilize a chemical treatment of the pulp and in particular, chlorine is frequently utilized for its bleaching efficiency. However, these chemical treatments create the problem of effluent disposal
Canadian Patent No. 758,488 to Jenness and Cooper describes a method of treating paper pulp with enzymes to improve the quality of the paper, although no mention is made of bleaching. USSR Pat. No. 321,563 to Grinberg et al and U.S. Pat. No. 3,962,033 to Eriksson et al describe methods respectively using enzymes and microorganisms to lower energy requirements during pulp refining. German Patent No. 3,110,117 to Eisenstein et al describes a somewhat similar process to that of Eriksson et al for pulping lignocellulosic material with white rot fungi, e.g. Pleurotus ostreatus. USSR Pat. No. 507,677 describes pretreatment of cellulosic raw material with a culture filtrate from wood decaying fungi to reduce sulphite pulping time and improve paper making properties. Swedish Patent No. 412,422 to Hartler describes a method of biological treatment of separated fibers from a first stage refining process which results in lower energy requirements in subsequent stages. Japanese Patent No. 10,240/82 to Oji Paper Company discloses mixing equal weights of pulp and mycelia of, for example, the mold Rhizopus javanicus to obtain paper. Canadian Patent No. 1,203,188 to Naylor et al discloses using a quinoid additive produced by a microorganism as a catalyst in the alkaline pulping of lignocellulose. French Patent No. 2,557,894 to Comtat et al discloses improving chemical pulp fibrillation by application of the enzyme xylanase.
Insofar as the mechanism is now understood, it is believed that when microorganisms and enzymes are used bleaching takes place due to lignin depolymerization. This may be catalyzed by the action of redox enzymes which may be produced in the action of a microorganism in the depolymerization of lignin.
Lignin degradation through the use of microorganisms has been studied, especially using white rot fungi such as Phanerochaet chrysosporium. The mechanism with Phanerochaete chrysosporium is now believed to include the rapid colonization of lignocellulosic fibers by hyphae through the lumens, followed by simultaneous degradation and removal of major wood components by extracellular biological reagents. A hydrogen peroxide dependent lignin peroxidase has been implicated as one enzyme involved in lignin degradation especially in the degradation of model compounds lignin. Other redox enzymes are probably also required for total mineralization. Active oxygen species (i.e. .sup.1 O.sub.2, OH.sup.., O.sub.2.sup.-)and some one-electron carriers, e.g. veratryl alcohol, may allow lignin degradation in the middle lamella, which presumably is inaccessible to enzymes originating in the lumen.
Kraft pulp can be partially delignified by Phanerochaete chrysosporium provided that the fungal treatment is followed by alkaline extraction or provided that the treatment is prolonged, e.g. ten days in unagitated cultures. Farrell, in U.S. Pat. No. 4,690,895, claims that ligninolytic enzymes present in extracellular growth medium from a fermentation of Phanerochaete chrysosporium can be used to bleach kraft pulp. However, hydrogen peroxide must be added, and the enzyme treatment is followed by alkaline extraction. It has also been found by others in the scientific literature that the enzyme xylanase aids delignification, presumably by liberating lignin from a complex with hemicellulose.
The physiology and enzymology of other white rot fungi may or may not be similar. It has now been found that other white rot fungi such as Coriolus and Pleurotus species are capable of lignin depolymerization. In particular, we have used Coriolus versicolor, a species isolated and assessed by us for the treatment of effluent from pulp and paper operations. A specimen of Coriolus versicolor has been deposited with the American Type Culture Collection as ATCC No. 20869.
It is an object of the invention to provide an improved process for bleaching in kraft pulp treatment.
It is another object of the invention to provide a process of bleaching kraft hardwood pulp using Coriolus versicolor.
It is another object of the invention to provide paper from kraft hardwood pulp that has been bleached through the action of Coriolus versicolor.